Correlation between metabolic cost and perceived comfort during human-in-the-loop optimization of a prosthetic foot

Human-in-the-loop optimization provides a standardized approach to enhance the effectiveness of assistive technology for gait by tuning devices to the needs of the individual user. Metabolic cost is frequently used as optimization objective. However, the assessment of metabolic cost requires relatively long-lasting physiological steady-state conditions. Subjective user preference could be an interesting alternative optimization objective that could shorten trial duration. We aim to explore whether the measured metabolic cost of walking correlates with the perceived comfort of the user during human-in-the-loop optimization process.

Ten transtibial amputees underwent an optimization protocol while walking on a treadmill with an experimental prosthetic foot with tuneable stiffness and alignment. We used human-in-the-loop optimization to minimize the metabolic cost of walking by optimizing the stiffness and alignment of the prosthetic foot, using an evolutionary optimization algorithm. The participant's perceived comfort of every optimization trial was determined via the visual analogue comfort scale. To determine the association between metabolic cost and perceived comfort during the optimization period, we performed a repeated measures correlation and a separate Pearson correlation for every individual.

Results showed a moderate negative correlation between metabolic cost and perceived comfort. On an individual level a significant correlation was found in six out of ten cases.

Perceived comfort allows people to prioritize different optimization objectives simultaneously, and apparently, metabolic cost is prioritized to varying extents.